1. Field of the Invention
The present invention relates to the art of earth boring with rolling cutter drill bits. In particular, this invention relates to an improved thrust bearing for sealed and lubricated three cone earth boring bits utilized for gas and oil well drilling.
2. Description of Related Art
Sealed and lubricated rolling cutter drill bits (also called rock bits) typically have three different bearing structures in each cutter. The first beating structure is designed to support cantilevered radial loads and is typically a journal bearing or a roller bearing. The second bearing structure is designed to retain the rolling cutter upon the cantilevered bearing spindle when the cutter is subjected to offward thrust. This retention system is generally comprised of either ball bearings or a friction bearing such as a snap ring or a threaded retaining ring. The third bearing structure is designed to carry onward axial thrust loads and is most often a friction type bearing. This thrust bearing in rolling cutter drill bits is the object of the present invention.
Analysis of used rolling cutter drilling bits shows that when high loads are combined with high rpm, the thrust bearing often fails or the resulting heat buildup causes degradation of the other bearings. Even though a great many designs and materials for rock bit thrust bearings have been used in an attempt to solve this problem, thrust bearing damage still remains a source of bearing failure, especially at very high rpm.
Friction thrust bearing elements in drill bits can be categorized as either fixed or floating bearing elements. Fixed elements are most often welded inlays or interference fitted inserts in the rolling cutter or in the end of the cantilevered bearing spindle. The most commonly used inlay is a STELLITE (registered trade mark) material welded on the thrust surface of the cantilevered bearing spindle. Another inlay features alternating surface areas of load bearing and anti-galling materials on the thrust surfaces as shown in U.S. Pat. Nos. 3,235,316 and 4,323,284. Alternately, a tungsten carbide button can be pressed into the cutter as shown in U.S. Pat. No. 3,384,426 or an irregularly shaped disk can be captured in the cutter as shown in U.S. Pat. No. 4,413,918. These materials must be fixed into the rolling cutter or the cantilevered bearing spindle because they lack either strength or ductility or both. For instance, the welded STELLITE and tungsten carbide inserts have less than 1% ductility. If they were not supported by the spindle substrate they would soon disintegrate in service. The ductile inlays have such low hardnesses and yield strengths they would soon wear and/or deform excessively inservice.
Drill bit performance in increasingly limited by thrust bearing capability as modem drilling applications demand ever higher loads and speeds. Degradation of thrust bearing surfaces via wear, spalling deformation, and fracture is primarily due to overheating. Significant decreases in both tribological and design strength properties with temperatures are well known to directly cause current materials systems failures.
Floating intermediate thrust bearing elements for drill bits are shown in U.S. Pat. Nos. 3,720,274; 4,410,284; 4,439,050 and 5,161,898. These floating thrust bearing elements have advantages over fixed elements well known and practiced in the prior art.
As shown in U.S. Pat. Nos. 4,439,050 and 5,161,898 and implied in several of the other patents cited above, floating bearing assemblies can be designed as one or more fixed bearing elements mounted or fused on carriers. The entire assembly functions as a single floating bearing element. These designs are not only more expensive to manufacture and occupy more bearing space than unitary material floating bearing elements in drill bits, they are also not as effective under extreme loads. This is because the bearing material can become separated from the carrier and contaminate the other bearing systems.
The magnitude of temperatures reached in the floating thrust elements of drill bits has been generally under-stated in the prior art. Floating thrust bearing elements in drill bits do not transfer heat into the external environment as easily as the cantilevered bearing spindle or the rolling cutter. Consequently, during a peak load event, the surface temperature of the thrust element will increase preferentially over its mating surfaces within the drill bit. After the load event, the heat still transfer relatively slowly from the thrust element through the mating surfaces and into the surrounding environment. The peak operating surface temperatures at the asperities of the floating thrust element can become extremely high, and in fact can exceed 1500F, leading to failure.
To function successfully in a drill bit, a unitary floating bearing element must have acceptable yield strength and ductility at operating temperature. In U.S. Pat. Nos. 3,721,307; 4,641,976 and 5,161,898, the minimum yield strength for a successful floating bearing element material in a drill bit is established at 140,000 psi and the minimum ductility is established to be about 1% to 4%. The Beryllium Copper of U.S. Pat. No. 3,721,307 is still in common use in drill bits, and is specified in ASTM B643-92, temper TH04 (HT). The minimum yield strength of this material is indicated in that specification to be from 140,000 psi to 155,000 psi. The copper based spinodal material of U.S. Pat. No. 4,641,976 with an ultimate tensile strength of 180,000 psi typically has a minimum yield strength of from 145,000 psi to 160,000 psi. Quoting U.S. Pat. No. 5,161,898 (Col. 1, line 47), xe2x80x9c. . . materials having minimum yield strengths of about 140,000 psi are needed to avoid macroscopic plastic deformation in service.xe2x80x9d It is understood that all these yield strength values are at room temperature. It is apparent in the prior art that 140,000 psi is common as the minimum room temperature yield strength for floating bearing elements in drill bits.
The copper based alloys of U.S. Pat. Nos. 3,721,307 and 4,641,976 mentioned above have a weakness, however. When the operating temperatures reach about 600 F. their strength greatly decreases. Generally, yield strengths of these copper alloys approach zero psi at 1500 F.
Historically, failure analysis of drill bit bearings has led bit designers to develop bearing materials with ever higher yield strengths. However, these materials probably performed better not because their room temperature yield strengths were higher, but because raising the room temperature yield strength tended to increase the operating temperature yield strengths. This success, coupled with the ease of measurement at room temperature, has led to a search for materials having high yield strength and good ductility at room temperature. Little attention has been given to materials with lower room temperature yield strength, but relatively high yield strength at operating temperature.
The present invention is directed at a new floating thus washer bearing element for rock bits formed of a material selected from the class of materials formed primarily of cobalt and chromium carbide and known as wrought, ductile STELLITES. The room temperature yield strength of these materials is less than 120,000 psi, much lower than what is shown in the prior art as necessary for a floating thrust bearing of a drill bit, making them an unexpected material choice for a floating thrust bearing. One material of this class is Aerospace Material Specification AMS 5894, a cobalt based, wrought super alloy commercially available by the trade name STELLITE, grade 6B, from Del Oro Stellite, Ontario, Canada. Floating trust washers made from this STELLITE material with a room temperature yield strength averaging 105,000 psi have been successfully field tested in multiple drilling environments. Under moderate drilling loads and rotary speeds, the washers exhibited good physical condition and minimal wear, similar to previous thrust bearing systems. Extensive testing under more severe loads and speeds demonstrated that the ductile STELLITE was a superior material for floating thrust bearings in rock bits.
It is an object of this invention to provide a sealed and lubricated rolling cutter drill bit with a floating bearing element made of ductile STELLITE having a room temperature tensile elongation greater than 4% and a room temperature yield strength less than 120,000 psi.
It is a further object of this invention to provide a sealed and lubricated rolling cutter drill bit with a floating thrust bearing element made of ductile STELLITE with a room temperature yield strength less than 120,000 psi, said material further having a yield strength greater than 30,000 psi, an un-notched longitudinal Charpy impact toughness of greater than 120 ft-lbs., and a tensile elongation greater than 10%, all at 1500 degrees F.
It is a still further object of this invention to provide a sealed and lubricated rolling cutter drill bit with a floating thrust bearing element made of a ductile, wrought STELLITE material.
It is a still further object of this invention to provide a sealed and lubricated rolling cutter drill bit with a floating thrust bearing element made of a ductile, wrought STELLITE with a room temperature yield strength less than 120,000 psi, and coated with a soft, metallic solid lubricant.